This, despite dwindling e-book and paperback revenues in the same period, is good news. Audiobook revenues have been consistently strong for several years, giving publishers a dependable non-print outlet. There is, however, always Amazon to contend with: a large distributor of audiobooks is Audible, an Amazon company, which publishers largely don't like dealing with. Other distributors, however, have been cropping up to deal with demand.

Regardless, it's a piece of good news for the publishing industry, which is all too familiar with small gains and big losses.